Class II major histocompatibility complex (MHC) molecules bind peptides within the endocytic compartment and the class II/peptide complex is expressed at the surface of antigen presenting cells (APC) for inspection by CD4+ T cells. This process is critical to the immune surveillance of the extracellular and endosomal spaces. The long term objective of this proposal is to elucidate the molecular mechanisms and intracellular pathways involved in the formation of class II/peptide complexes within APC. As an approach, EBV-transformed B cell lines (B-LCL) are used as model APC to take advantage of the opportunity they afford to use somatic cell genetics in the dissection of a multi-step pathway. One interesting group of mutant B-LCL are defective in class II-restricted antigen presentation, despite expression of normal class II genes. In these mutants, the majority of class II molecules fail to become loaded with the normal spectrum of naturally processed peptides and are instead associated with fragments of the invariant chain, a class II molecular chaperone. The genetic basis of the defect in these mutants is mutation of the HLA-DMB gene, an MHC linked, relatively non-polymorphic class II-like gene. These results indicate that HLA-DM plays an essential role in the class II antigen presentation pathway in B-LCL, although its specific function is unknown. The primary goal of this proposal is to determine the mechanism of action of HLA-DM in class II restricted antigen presentation. The specific aims are: (1) to determine whether HLA-DM expression alters trafficking of conventional class II MHC molecules in B lymphoblastoid cells; (2) to investigate the hypothesis that HLA-DM catalyzes release of invariant chain fragments and/or peptide-loading of conventional class II molecules, such as HLA-DR; (3) to determine if HLA-DM directly interacts with conventional class II molecules; (4) to investigate the basis of the temperature sensitive phenotype of a unique DMB mutant The approaches center on using mutant cell lines in biochemical, histological and cell biological assays. In addition, molecular genetic techniques will be used to generate new mutants to investigate the hypothesis that DR and DM directly interact. These studies should contribute to an understanding of a critical step in the class Il antigen presentation pathway and thus may have implications for diseases which involve class II-restricted T cells and for efforts to augment or dampen immune responses in humans.